We currently size the msi window trap page according to the host's page
size so that we poke a working hole into a memory slot in case we overlap.

However, this is only ever necessary with KVM active. Without KVM, we should
rather try to be host platform agnostic and use a constant size: 4k.

This fixes a build breakage on win32 hosts.
Signed-off-by: NAlexander Graf <agraf@suse.de>